Coincidentally, the Typepad outage also caused me to think about this. The problem with Typepad's backup facility is that it doesn't seem to work very well, and only deals with the text part of blog content. If I were to backup my Typepad blog and relocate the content to a new server I would also have to separately move all the image and attached files and then edit the backup file to reflect the new target url, and then I would still rely on hope that the new server would import all of my content reasonably well (which Wordpress has yet to be able to do).
What we need is an imaging facility that copies an entire blog and everything associated with it from one server to another. I did get an email this am from a guy who is building just such a service, I hope I will be able to take it for a spin to see how well it works.
With these three things, in addition to a common export format, even in an extended outage, the user would be in full control of his or her content. Users should insist on this level of support from vendors, or take their business to vendors who provide it.